The answer to this can be found in Chapter 1, pages 46 and 47 in the paperback edition of the book. There, Diamond provides us with a long list of large animals which existed in North America prior to 11,000 BC, which is around the time that human beings are believed to have started to inhabit the Americas.

Diamond mentions that the American West would have looked much like the Serengeti Plains in Africa do today. He says that there were elephants and horses, lions and cheetahs, camels and giant ground sloths. He also mentions Harrington's mountain goats. These are all of the animals that he lists as having existed in North America prior to the extinctions that (he believes) were caused by human actions.